MONITORING UPTAKE AND CONTENTS OF MG, CA AND K IN NORWAY SPRUCE AS INFLUENCED BY PH AND AL, USING MICROPROBE ANALYSIS AND STABLE-ISOTOPE LABELING

In a model system using intact spruce trees (Picea abies [L.] Karst.) we followed the path of magnesium, calcium and potassium during uptake into the root and during long-range transport into the shoot, by mult iple stable isotope labelling. The roots of two- and three-year-old sp ruce trees originating from soil culture were removed from the soil an d, in part or in tote, exposed to labelling solutions containing the s table isotopes Mg-25 or Mg-26, K-41 and Ca-42 or Ca-44. Optical-emissi on-spectroscopy (ICP-OES) of plant fractions and labelling solutions w as combined with the quantitative analysis of stable isotope ratios in sections of shock frozen, cryosubstituted material using the laser-mi croprobe-mass-analyser (LAMMA). This combination allowed us to disting uish, both in bulk samples and on the cellular level between (i) the f raction of elements originally present in the plant before the start o f the labelling, (ii) the material taken up from the labelling solutio n into the plant and (iii) any material released by the plant into the labelling solution. In single-root labelling experiments, roots of th ree-year-old spruce trees, grown in nursery soil, were exposed to vari ous pH conditions. The exchange of Mg and Ca with the labelling soluti on was nearly 100% in the cell walls of the mycorrhized finest roots. This exchange was only slightly affected by a step down to pH 3.5. The absolute Mg and Ca content in the cell walls was moderately reduced b y incubation at pH 3.5 and strongly reduced in the presence of Al at t his pH. After a pH 3.5 and 2 mM Al treatment we found Al in the xylem cell walls and the cortex cell lumina at elevated concentrations. To a nalyse the combined effect of high Al and high proton concentrations o n the long-range transport, we used a ''split-root system''. The root mass of an intact two-year-old spruce tree, grown in mineral soil, was divided into even parts and both halves incubated in solutions with t wo sets of different stable isotopes of Mg and Ca (side A: no Al, Mg-2 5 and Ca-42; side B: +Al, Mg-26 and Ca-44) and K-41 on both sides. We observed a large uptake of Mg, Ca and K into the plant and a pronounce d release. The net uptake of all three elements was lower from the Al- doted solution. In cross-sections of the apical shoot we found after a seven-day labelling period about 60-70% of the Mg and Ca and 30% of t he K content in the xylem cell walls originating from both labelling s olutions. The clear majority of the Mg and Ca label originated from th e Al-doted side.